Autres pays, autres moeurs: in Ireland it's considered deeply rude and attention-seeking to join in with a singer, except *in certain songs* with the chorus; it's not even considered polite to join in the chorus in many songs. Mostly the singer will call out "Sing up" after a line or two of the chorus, if she or he wishes others to join in that chorus, and then and only then will listeners join, falling silent again to allow the singer room for the verses as a solo.